Excessive osteoclast activity can lead to pathological bone resorption, which is a serious consequence of many clinical diseases including, osteoporosis, metastastic bone disease and periodontal disease. Therefore, current therapies such as bisphosphonates and denosumab (anti-RANKL antibody) have focused on inhibiting osteoclast function. Furthermore, it is becoming clear that osteoclasts are required for overall bone remodeling, and therapeutic elimination may cause more harm than good. Therefore, to improve treatment outcomes, understanding the cellular signaling networks involved in bone remodeling process is important to identifying more effective therapeutic targets. BMPs, which are currently used therapeutically to promote bone healing and regeneration, have been shown by us and others to act synergistically with RANKL, the main cytokine responsible in activating osteoclasts, to enhance osteoclastogenesis. BMPs can signal through both noncanonical MAPK signaling and canonical Smad 1/5 signaling. Previous data from our lab has shown that phospho-SMAD 1/5 expression increases in osteoclasts at the time of fusion of mononuclear precursors into multinucleated osteoclasts. I have generated mice that are conditionally deleted for Smad 1 and 5 in osteoclasts using the Cathepsin-Cre mice to test the hypothesis that Smad 1/5 expression is necessary for osteoclast fusion and activity. Secondly, I have generated preliminary data demonstrating that the expression of myosin X, an unconventional myosin responsible for regulating the sealing zone patterning in osteoclasts, increases with BMP2 stimulation of osteoclasts and is a downstream target of Smad 1/5. At the completion of my proposed experiments I expect to better understand osteoclast differentiation, particularly during osteoclast fusion and thereby uncover potential novel therapeutic targets that can be used to inhibit osteoclast function.